Optical emission spectroscopy (or OES for short) means in general a measurement method in which atoms and molecules of a target material are provided with a sufficient stimulus that causes said particles to be excited into certain excited states, the spontaneous relaxation of which causes detectable emission in the optical wavelengths of the spectrum of electromagnetic radiation.
FIG. 1 illustrates an OES arrangement that is known from a prior art patent publication DE 38 40 106. A measuring head or test probe 101 is brought into contact with an electrically conductive target material 102. A high voltage is applied between the target material 102 and an electrode 103 within the measuring head, thus causing an electric arc to be ignited therebetween. The energy involved in the electric discharge causes atoms and molecules of the target material 102 to be evaporized into the chamber-like space 104 that surrounds the lower end of the electrode 103, where the excited evaporated particles constitute a plasma. Spontaneous relaxation of the excited states of the particles cause electromagnetic radiation in the optical range to be emitted. A part of the emitted radiation travels through a collimator 105 and hits a deflecting mirror 106, which directs the radiation through a slit 107 onto a wavelength-dispersive focusing mirror 108 and further to a detector 109. Knowing the characteristics of the optical system it is possible to deduce the wavelength of a certain part of the incident radiation by noting the spot at which it hit the detector 109.
A major area of application for OES measurements is in the field of metallurgy, where OES analyzers are frequently used both in laboratory and on-site conditions for purposes like sorting, material control and process management. Carbon is an important element in this respect because of its alloying properties, and in many OES measurements it would be especially advantageous if carbon could be measured reliably. The best analytical spectral line of carbon corresponds to emitted radiation at a wavelength of 193.090 nm in vacuum; this spectral line is commonly referred to as the “193 nm line” of carbon. However, it has been commonly regarded as impossible to measure it with an OES arrangement where the arc chamber is not isolated from the ambient air. Optical emission spectrometers for measuring the 193 nm line of carbon are commercially available, but they have the common feature that they require the test probe to be flushed with an inert gaseous medium, usually argon. Additionally there have been problems with lightguides: the transmissivity of known optical fiber lightguides tends to drop dramatically at wavelengths below about 200 nm.
The problems of detecting the 193 nm line of carbon in air have been addressed for example on page 310 of K. Slickers: “Automatic-Emission-Spectroscopy”, Bruhlsche Universitetsdruckerei, Giessen, Germany, 2nd edition, 1993, which is widely regarded as the most authoritative monographic volume in this technological field. On the same page the author suggests that measurements with argon-flushed test probes are the most likely way to successive on-site detection.
The relative ease of measuring the 193 nm line of carbon with an argon-flushed test probe has resulted in a situation where all commercially available on-site OES measurement devices use argon flushing. Although it enables the measurement to succeed, the requirement for carrying a pressurized container of argon around is a major disadvantage that limits the usability of OES arrangements.
An alternative approach to the OES measurement of carbon is to use the molecular, so-called CN emission bands, of which the one having a wavelength of 387.1 nm is most readily available. Such an approach has been described for example in N. N. Sorokina and P. A. Kondrat'ev: “A Spectral Method for Determining Carbon by Cyanogen Bands” (russ.), Zavodskaja Lab. 31 (1964), pp. 1344-1345. The CN emission bands do not facilitate obtaining as exact and unequivocal measurement results as would the atomic emission line of 193 nm.